Because of their high volumetric efficiency and thus their small size, multilayer ceramic capacitors (MLC's) are the most widely used form of ceramic capacitors. These capacitors are fabricated by stacking and cofiring thin sheets of ceramic dielectric on which an appropriate electrode pattern is printed. Each patterned layer is offset from the adjoining layers in such manner that the electrode layers are exposed alternately at each end of the assemblage. The exposed edges of the electrode pattern are coated with a conductive material which electrically connects all the layers of the structure, thus forming a group of parallel connected capacitors within the laminated structure. Capacitors of this type are frequently referred to as monolithic capacitors.
The thin sheets of ceramic dielectric used for the fabrication of MLC's are comprised of a layer of finely divided dielectric particles which are bound together by an organic polymeric material. The unfired ceramic can be prepared by slip casting a slurry of the dielectric particles dispersed in a solution of polymer, plasticizer and solvent onto a carrier such as polypropylene, Mylar.RTM. polyester film or stainless steel and then adjusting the thickness of the cast film by passing the cast slurry under a doctor blade to form a thin "green tape".
Metallizations useful in producing conductors for multilayer capacitors normally comprise finely divided metal particles applied to green tape in the form of a dispersion of such particles in an inert liquid vehicle. Although the above-described "green tape" process is more widely used, there are nevertheless other procedures with which dielectric compositions of the invention can be used to make MLC's. One technique is the so-called "wet process". In one aspect, this may involve passing a flat substrate through a falling sheet of dielectric slip one or more times to build up a dielectric layer (see Hurley et al., U.S. Pat. No. 3,717,487).
Another method of making MLC's involves forming a paste of the dielectric material and then alternately screen printing the dielectric and metal layers with intervening drying steps until the designed structure is complete. A second electrode layer is then printed atop the dielectric layer(s) and the entire assemblage is cofired.
Monolithic multilayer capacitors are typically manufactured by cofiring barium titanate based formulations and conductive electrode materials in oxidizing atmospheres at temperatures of 1200.degree.-1400.degree. C. This process yields durable, well sintered capacitors with high dielectric constant, e.g., greater than 1000. However, firing under these conditions requires an electrode material with high melting point, good oxidation resistance at elevated temperatures, sinterability at the maturing temperature of the dielectric, and minimal tendency to interact with the dielectric at the sintering temperature. These requirements normally limit the choice of electrode materials to the noble metals platinum and palladium, or to alloys of platinum, palladium and gold. See also U.S. Pat. No. 3,872,360 to J. L. Sheard which is directed to the preparation of monolithic multilayer capacitors.
Several attempts have been made to reduce the maturing temperature of dielectrics by the use of "sintering aids". Additions of bismuth oxide or bentonite to barium titanate lowers the maturing temperature to about 1200.degree. C. (Nelson et al. U.S. Pat. No. 2,908,579). Maturing temperatures of 1200.degree.-1290.degree. C. may be attained by addition of phosphates to titanates as described in Thurnauer et al. U.S. Pat. No. 2,626,220. However, in each of these cases, the decrease in maturing temperature is not sufficient to permit the use of cofired silver electrodes, and dielectric properties are often degraded.
Another technique for lowering the sintering temperature of titanate-based dielectrics is by mixing high temperature ferroelectric phases (titanates, zirconates, etc.) with glasses which mature at relatively low temperatures. Examples of this approach are given in Maher U.S. Pat. No. 3,619,220, Burn U.S. Pat. No. 3,638,084, Maher U.S. Pat. No. 3,682,766, and Maher U.S. Pat. No. 3,811,937. The drawback of this technique is that the dilution effect of the glass often causes the dielectric constant of the mixture to be relatively low.
Bouchard has very successfully approached the problem of dielectric compositions having low firing temperatures and dielectric constants as high as 6000 for use in Z5U-type capacitors by the use of lead-based dielectrics instead of barium titanate. These substituted lead titanate compositions correspond to the following formula:
______________________________________ (Sr.sub.x Pb.sub.1-x TiO.sub.3).sub.a (PbMg.sub.r W.sub.s O.sub.3).sub.b, wherein ______________________________________ x = 0-0.10 r = 0.45-0.55 a = 0.35-0.5 s = 0.55-0.45 b = 0.5-0.65 .SIGMA.(r + s) = 1, and .SIGMA.(a + b) = 1. ______________________________________
Such materials are disclosed in U.S. Pat. Nos. 4,048,546, 4,063,341, and 4,228,482, all to Bouchard.
More recently, Thomas in U.S. continuing patent application Ser. No. 713,099, filed Mar. 18, 1985 and allowed Sept. 12, 1985, has improved upon the Bouchard dielectric compositions to make them more suitable for Z5U-type service. In these, the Bouchard compositions are doped with small amounts of transition metal oxides and zirconates and stannates of cadmium and zinc.
Notwithstanding the substantial progress toward attaining higher dielectric constants, the electronics industry foresees the need for dielectric compositions with low PbO contents (i.e. &lt;10% wt.) having still higher dielectric constants (K) on the order of 8000 and even higher, which nevertheless can still be used with conventional silver-containing electrodes such as 30/70 palladium/silver electrodes.